Prince Charles and Diana, Princess of Wales, formally separated in December 1992, but it took almost four years to finalize the much-disputed divorce. Their lawyers haggled over money and access, but one of the most painful disagreements involved Diana's title. That's when Prince William–just barely a teenager—tried to comfort his wounded mom.

As the mother of the future heir, the divorcee wanted to keep the styling "Her Royal Highness." Even Queen Elizabeth II agreed to the condition, but Charles was "adamant" about denying his ex-wife the privilege, according to the New York Times.

The loss of "HRH" meant Diana would technically curtsey to her own children, as well as Charles. Although she'd remain a member of British nobility, the Princess lost her formal connection to the royal family, including her sons.

'"She told me how he had sat with her one night when she was upset over the loss of HRH, put his arms around her and said, 'Don't worry, Mummy. I will give it back to you one day when I am king,'" he wrote.

Over two decades later, it's no surprise the future monarch made such a sweet promise. Diana would pass away only a year later, but Prince William has continued to honor her legacy ever since. Now more than ever, the Duke of Cambridge speaks out about his loss. "I still miss my mother every day," he said recently. "And it's 20 years after she died."